Agricultural drainage ditches act as key phosphorus (P) transport pathways to local water bodies. Ditch soils may act as either a sink or source of P to overlying waters. We describe a two-part investigation of P and P dynamics within select drainage ditches in the Atlantic Coastal Plain in Maryland, USA.
1) Knowledge of the vertical and lateral distribution of P within ditch soils is important in order to understand P transport and retention mechanisms and to develop best management practices to minimize P losses. The objectives of this study were to assess the 1) vertical distribution of soil P pools as a function of depth and horizonation and 2) lateral distribution of surficial ditch soil P. The study site was at the University of Maryland Eastern Shore Research Farm, Princess Anne, Maryland, USA. Twenty-one profiles were sampled and analyzed for total P, water-extractable P, Mehlich 3-P, acid ammonium oxalate-extractable P, Fe, and Al (Pox, Feox, Alox), pH, organic C, and sulfides (which are present in surface and subsurface ditch soils at this site). Surface soils (0-5 cm) were sampled at 10-m intervals and analyzed for Pox, Feox, Alox and pH. The most significant effects on the vertical distribution of P fraction concentrations and sorption capacity were due to pedological differences between horizons resulting from variations in pedogenic processes including gleization and humification. Total P (n = 126) in all horizons sampled ranged from 27 to 4882 mg kg-1. Organic soil horizons were highest in Pox, Feox, and Alox. A horizons formed in alluvium were greater in Pox, Feox, and Alox than subsurface horizons formed in sandy Coastal Plain sediments. Organic-enriched alluvial A horizons had greater concentrations than did gleyed A horizons. Surficial ditch soil P and sorption capacity were found to be laterally autocorrelated. Oxalate-extractable P (mean= 700 mg kg-1) exhibited a high standard deviation across the study area (overall 580 mg kg-1) and within individual ditches (maximum 1383 mg kg-1). Distinct areas of either low or high Pox were observed within three ditches. Phosphorus was well correlated with Alox or Feox within specific ditches, but was not well correlated across all ditches.
2) The above investigation is being followed by a detailed inspection of the P pools of two ditches. Soils, vegetation, and micro flora/fauna are analyzed for P contents on a seasonal basis while parallel measurements of P exports and water quality variables (pH, temp, dissolved oxygen concentration, and ditch soil redox potential) are conducted. The objectives of this study are to a) quantify the magnitude of and seasonal dynamics of P in surface and subsurface soil horizons, active and senesced vegetation, and microbial and algal pools; b) tie these dynamics to P export trends; and c) characterize the role that ditch organic matter and surficial sulfidic materials have on P transport. This study is ongoing; preliminary results will be presented.